Abstract: [Objective] The effects of different types of acid rain on the fine roots of Quercus acutissima were studied to provide a theoretical basis for studying the impacts of the pollution by acid rain on forest ecosystem and forest management and sustainable development.[Method] A total of 120 sample plots in a size of 2 m by 0.6 m were selected randomly 1.0 m away from the trunks of Q. acutissima trees, the pH of the 0-20 cm surface soil was 4.25, exchangeable Al³⁺ and H⁺ contents were 4.90 and 0.30 cmol·kg^-1. Three types of acid solutions at 5:1, 1:1,and 1:5 of sulfur to nitrogen ratios were prepared according to the molar mass ratio, and diluted into a gradient of three pH of 4.5, 3.5 and 2.5 in the field with the water from mountain stream, which will be used to simulate different types of acid rain. 2/3 of the monthly average rainfall in the 2002-2003 period in Nanjing was set as the total annual addition of acid rain, and then the monthly spraying amount of the acid solution was obtained according to the monthly rainfall ratio. From March 2015 to February 2016, the acid solutions were sprayed twice a month in the corresponding sample plots (half of the total each spray, not block the natural rainfall). Soil samples were collected every three months, 30 sample plots were sampled each time (sample plots were discarded after sample collection). Samples of 0-20 cm soil column were taken with soil drill, and roots of Q. acutissima with diameters < 2 mm were picked out and measured for all traits.[Result] When the pH of added acid rain was 4.5, the biomass of fine roots increased, the proportion of living roots increased, the length density increased, the number of root tips increased, the average root diameter decreased and the turnover rate accelerated. When the pH of acid rain was less than 4.5, the total biomass of fine root decreased, the proportion of living roots decreased, the length density decreased, the number of root tips decreased, the average root diameter increased, and the turnover rate decreased. There was a significant interaction between the pH and sulfur to nitrogen ratio on the effect of acid rain on fine roots. With the decrease of pH, the difference of different sulfur to nitrogen ratio treatments decreased gradually. The root biomass, root length, root density, root tip density and turnover rate were 5:1 > 1:1 > 1:5 and the average root diameter were 5:1 < 1:1 < 1:5 with the same acidity and different sulfur to nitrogen ratio treatments. The extents of responses of total root biomass, root length density, root tip density to acid rain were in the order:summer > spring > autumn > winter. The extents of responses of the average root diameter of the fine roots to acid rain were in the order:winter, spring > autumn, summer, there were no significant differences among seasons in the proportion of fine roots to living roots ratio.[Conclusion] The response of fine roots to acid rain varied among the seasons, with the decrease of pH of acid rain, the effect of acid rain on the growth of fine roots showed promotion first followed by suppression. The effect of acid rain on the growth of fine roots was mainly limited to the very small diameter roots (first and second order roots). Interaction was found between the impacts of pH of the acid rain and the sulfur to nitrogen ratio on fine roots, with the decrease of pH, the effect of sulfur to nitrogen ratio of acid rain gradually decreased. The acidification stress of N was stronger than S, with the decrease of sulfur to nitrogen ratio of acid rain, i.e. the increase of nitrogen, was not favorable to the growth of fine roots.

Key words: Quercus acutissima stand, acid rain, sulfur and nitrogen ratio, pH-value, fine root growth